implement true throttling

README.md

@@ -73,6 +73,11 @@ Type: `Number|String` Default: `250`
 
 The throttle is managed by an internal function that prevents performance issues from continuous firing of `window.onscroll` events. Using a throttle will set a small timeout when the user scrolls and will keep throttling until the user stops. The default is `250` milliseconds.
 
+#### debounce
+Type: `Boolean` Default: `true`
+
+By default the throttling function is actually a [debounce](http://underscorejs.org/#debounce) function so that the checking function is only triggered after a user stops scrolling. To use traditional throttling where it will only check the images every `throttle` milliseconds, set `debounce` to `false`.
+
 #### unload
 Type: `Boolean` Default: `false`
 

dist/echo.js

@@ -1,7 +1,9 @@
 /*! echo.js v1.6.0 | (c) 2014 @toddmotto | https://github.com/toddmotto/echo */
 (function (root, factory) {
   if (typeof define === 'function' && define.amd) {
-    define(factory);
+    define(function() {
+      return factory(root);
+    });
   } else if (typeof exports === 'object') {
     module.exports = factory;
   } else {
@@ -15,16 +17,22 @@
 
   var callback = function () {};
 
-  var offset, poll, throttle, unload;
+  var offset, poll, delay, useDebounce, unload;
 
   var inView = function (element, view) {
     var box = element.getBoundingClientRect();
     return (box.right >= view.l && box.bottom >= view.t && box.left <= view.r && box.top <= view.b);
   };
 
-  var debounce = function () {
+  var debounceOrThrottle = function () {
+    if(!useDebounce && !!poll) {
+      return;
+    }
     clearTimeout(poll);
-    poll = setTimeout(echo.render, throttle);
+    poll = setTimeout(function(){
+      echo.render();
+      poll = null;
+    }, delay);
   };
 
   echo.init = function (opts) {
@@ -41,16 +49,17 @@
       l: optionToInt(opts.offsetLeft, offsetHorizontal),
       r: optionToInt(opts.offsetRight, offsetHorizontal)
     };
-    throttle = optionToInt(opts.throttle, 250);
+    delay = optionToInt(opts.throttle, 250);
+    useDebounce = opts.debounce !== false;
     unload = !!opts.unload;
     callback = opts.callback || callback;
     echo.render();
     if (document.addEventListener) {
-      root.addEventListener('scroll', debounce, false);
-      root.addEventListener('load', debounce, false);
+      root.addEventListener('scroll', debounceOrThrottle, false);
+      root.addEventListener('load', debounceOrThrottle, false);
     } else {
-      root.attachEvent('onscroll', debounce);
-      root.attachEvent('onload', debounce);
+      root.attachEvent('onscroll', debounceOrThrottle);
+      root.attachEvent('onload', debounceOrThrottle);
     }
   };
 
@@ -88,9 +97,9 @@
 
   echo.detach = function () {
     if (document.removeEventListener) {
-      root.removeEventListener('scroll', debounce);
+      root.removeEventListener('scroll', debounceOrThrottle);
     } else {
-      root.detachEvent('onscroll', debounce);
+      root.detachEvent('onscroll', debounceOrThrottle);
     }
     clearTimeout(poll);
   };

dist/echo.min.js

@@ -1,2 +1,2 @@
 /*! echo.js v1.6.0 | (c) 2014 @toddmotto | https://github.com/toddmotto/echo */
-!function(t,e){"function"==typeof define&&define.amd?define(e):"object"==typeof exports?module.exports=e:t.echo=e(t)}(this,function(t){"use strict";var e,o,n,r,c={},i=function(){},a=function(t,e){var o=t.getBoundingClientRect();return o.right>=e.l&&o.bottom>=e.t&&o.left<=e.r&&o.top<=e.b},d=function(){clearTimeout(o),o=setTimeout(c.render,n)};return c.init=function(o){o=o||{};var a=o.offset||0,l=o.offsetVertical||a,u=o.offsetHorizontal||a,f=function(t,e){return parseInt(t||e,10)};e={t:f(o.offsetTop,l),b:f(o.offsetBottom,l),l:f(o.offsetLeft,u),r:f(o.offsetRight,u)},n=f(o.throttle,250),r=!!o.unload,i=o.callback||i,c.render(),document.addEventListener?(t.addEventListener("scroll",d,!1),t.addEventListener("load",d,!1)):(t.attachEvent("onscroll",d),t.attachEvent("onload",d))},c.render=function(){for(var o,n,d=document.querySelectorAll("img[data-echo]"),l=d.length,u={l:0-e.l,t:0-e.t,b:(t.innerHeight||document.documentElement.clientHeight)+e.b,r:(t.innerWidth||document.documentElement.clientWidth)+e.r},f=0;l>f;f++)n=d[f],a(n,u)?(r&&n.setAttribute("data-echo-placeholder",n.src),n.src=n.getAttribute("data-echo"),r||n.removeAttribute("data-echo"),i(n,"load")):r&&(o=n.getAttribute("data-echo-placeholder"))&&(n.src=o,n.removeAttribute("data-echo-placeholder"),i(n,"unload"));l||c.detach()},c.detach=function(){document.removeEventListener?t.removeEventListener("scroll",d):t.detachEvent("onscroll",d),clearTimeout(o)},c});
+!function(t,e){"function"==typeof define&&define.amd?define(function(){return e(t)}):"object"==typeof exports?module.exports=e:t.echo=e(t)}(this,function(t){"use strict";var e,n,o,r,c,i={},l=function(){},a=function(t,e){var n=t.getBoundingClientRect();return n.right>=e.l&&n.bottom>=e.t&&n.left<=e.r&&n.top<=e.b},d=function(){(r||!n)&&(clearTimeout(n),n=setTimeout(function(){i.render(),n=null},o))};return i.init=function(n){n=n||{};var a=n.offset||0,u=n.offsetVertical||a,f=n.offsetHorizontal||a,s=function(t,e){return parseInt(t||e,10)};e={t:s(n.offsetTop,u),b:s(n.offsetBottom,u),l:s(n.offsetLeft,f),r:s(n.offsetRight,f)},o=s(n.throttle,250),r=n.debounce!==!1,c=!!n.unload,l=n.callback||l,i.render(),document.addEventListener?(t.addEventListener("scroll",d,!1),t.addEventListener("load",d,!1)):(t.attachEvent("onscroll",d),t.attachEvent("onload",d))},i.render=function(){for(var n,o,r=document.querySelectorAll("img[data-echo]"),d=r.length,u={l:0-e.l,t:0-e.t,b:(t.innerHeight||document.documentElement.clientHeight)+e.b,r:(t.innerWidth||document.documentElement.clientWidth)+e.r},f=0;d>f;f++)o=r[f],a(o,u)?(c&&o.setAttribute("data-echo-placeholder",o.src),o.src=o.getAttribute("data-echo"),c||o.removeAttribute("data-echo"),l(o,"load")):c&&(n=o.getAttribute("data-echo-placeholder"))&&(o.src=n,o.removeAttribute("data-echo-placeholder"),l(o,"unload"));d||i.detach()},i.detach=function(){document.removeEventListener?t.removeEventListener("scroll",d):t.detachEvent("onscroll",d),clearTimeout(n)},i});

src/echo.js

@@ -1,7 +1,7 @@
 (function (root, factory) {
   if (typeof define === 'function' && define.amd) {
     define(function() {
-      return factory(root)
+      return factory(root);
     });
   } else if (typeof exports === 'object') {
     module.exports = factory;
@@ -16,16 +16,22 @@
 
   var callback = function () {};
 
-  var offset, poll, throttle, unload;
+  var offset, poll, delay, useDebounce, unload;
 
   var inView = function (element, view) {
     var box = element.getBoundingClientRect();
     return (box.right >= view.l && box.bottom >= view.t && box.left <= view.r && box.top <= view.b);
   };
 
-  var debounce = function () {
+  var debounceOrThrottle = function () {
+    if(!useDebounce && !!poll) {
+      return;
+    }
     clearTimeout(poll);
-    poll = setTimeout(echo.render, throttle);
+    poll = setTimeout(function(){
+      echo.render();
+      poll = null;
+    }, delay);
   };
 
   echo.init = function (opts) {
@@ -42,16 +48,17 @@
       l: optionToInt(opts.offsetLeft, offsetHorizontal),
       r: optionToInt(opts.offsetRight, offsetHorizontal)
     };
-    throttle = optionToInt(opts.throttle, 250);
+    delay = optionToInt(opts.throttle, 250);
+    useDebounce = opts.debounce !== false;
     unload = !!opts.unload;
     callback = opts.callback || callback;
     echo.render();
     if (document.addEventListener) {
-      root.addEventListener('scroll', debounce, false);
-      root.addEventListener('load', debounce, false);
+      root.addEventListener('scroll', debounceOrThrottle, false);
+      root.addEventListener('load', debounceOrThrottle, false);
     } else {
-      root.attachEvent('onscroll', debounce);
-      root.attachEvent('onload', debounce);
+      root.attachEvent('onscroll', debounceOrThrottle);
+      root.attachEvent('onload', debounceOrThrottle);
     }
   };
 
@@ -89,9 +96,9 @@
 
   echo.detach = function () {
     if (document.removeEventListener) {
-      root.removeEventListener('scroll', debounce);
+      root.removeEventListener('scroll', debounceOrThrottle);
     } else {
-      root.detachEvent('onscroll', debounce);
+      root.detachEvent('onscroll', debounceOrThrottle);
     }
     clearTimeout(poll);
   };